Analysis of trace elements including heavy metals has been performed for material circulation research in the ocean, water quality testing of tap water, water quality testing of lake water and river water, and the like. At the time of analysis of trace elements, for the purpose of separating the trace elements from interference components or for the purpose of improving the sensitivity of analysis, there may be required the separation and concentration of the trace elements based on solid-phase extraction using a chelating resin, and the like. When the separation and concentration are performed, it is required to adjust in advance a liquid sample to optimum pH conditions.
Further, in view of the purposes of analysis of trace elements, it is desirable that contamination factors in the pH adjustment are removed as much as possible. In this regard, the present inventors have developed an indirect measurement method using color change characteristics of a pH indicator (Non Patent Literature 1). In the indirect measurement method, an optical sensor is used as a pH measuring instrument, and hence contamination due to contact between the measuring instrument and the sample can be eliminated. Further, in the indirect measurement method, methyl yellow, methyl orange or methyl red is used as the pH indicator, and ammonia is used as a pH adjusting liquid. In the indicator and the adjusting liquid which are used for the indirect measurement method, contamination due to the indicator and the adjusting liquid can be eliminated by using a high purity reagent in which the amount of metal components is suppressed to be sufficiently small.